As a press, a variety of machines are employed depending on the kinds of plastic workings and they are broadly broken down into a hydraulic press using fluid pressure of oil, water, air or the like, and a mechanical press using mechanical force.
In the case of a conventional press for forming semiconductor packages, generally, the press unit for punching lead frames is driven by an oil hydraulic cylinder, while the feeding mechanism for feeding operation is driven by an air hydraulic cylinder.
The press driven by an oil hydraulic cylinder can be provided in a relatively simple structure by use of an oil hydraulic cylinder, various valves and the like, but often causes a problem of oil leaking from the cylinder during operation and moreover causes hardness of hearing to field operators due to big noise generated at the time of switching valves or noise of punching because as high a pressure as 100 kg/cm.sup.2 or higher is employed. In addition, the punching rate of around 60 punchings per minute is too slow to keep pace with the recent trend of massive and speedy production of semiconductors. Therefore, use of a conventional oil hydraulic press to meet the production demand would require a multitude of equipments and a number of persons to operate the equipments, whereby not only a large space out of a semiconductor plant need to be alloted but also large economic loss due to the elated installation costs and lowered productivity would results.
And a feeding mechanism for feeding lead frames as workpieces is driven by a pneumatic cylinder which is operated discretely from and at a markedly higher speed than the oil hydraulic cylinder for driving the press. This discrete use of driving sources of an oil hydraulic device and a pneumatic device acts as a hindrance in improving throughput of trimming and forming process. Further, reliance in operation is hardly secured in the case that a pneumatic feeding device is used in connection with other devices using pneumatic pressure, because the air supply fluctuates widely in pressure and in flow rate in accordance with the number of pneumatic devices used in the production process.
To cope with the above described problems, there was proposed a mechanical press driven by a servomotor and a method for controlling the same in Korean unexamined patent publication 89-700079, which press is so arranged as to acquire punching stroke by turning the rotating motion of a servomotor into a reciprocating motion of a ram through a mechanical power transmission mechanism, and is able to minimize noises generated in a press by reducing the speed of a ram below a certain level at the instant of impact on a workpiece by a tool which is attached on the tip of ram through controlling the rotating speed and direction of the servomotor, whereby there is realized an advantage that major problems of an oil hydraulic press such as decreased processing speed and low productivity are resolved.
However, a press as described above had a problem that a lot of time loss and economic loss are encountered from maintenance and repair work in an effort to prevent service life shortage of a servomotor which results from overload and overheat owing to excessively frequent switchings of the rotational direction of a servomotor. And, as rotational switching needs to go through a step of pause, another problem is posed that high productivity is not expected due to a longer operating time consumed for a single stroke.
Besides, there was not solved the problem of reduction in productivity and reliability of operation of the feeding device due to the difference between the speeds of the driving source for the press and the driving source for the feeding device, caused by use of a pneumatic device for the feeding device apart from the press, as described previously.